Solid state imaging sensors are used in a wide variety of consumer electronics products such as smart phones, tablet computers, laptop computers, as well as desktop units. A typical imaging sensor is an integrated circuit die that has an array of photo sensors and is positioned at a focal plane of an optical (lens) system that is to be aimed at a scene to be captured. When a user signals a shutter release, namely that a picture is to be taken, sensor controller circuitry will reset the sensor array and then allow it to respond to incident light from the scene. A conventional higher resolution imaging sensor has millions of photo sensors, and relatively complex analog signal conditioning and routing circuitry that serves to collect all of the photo sensor analog signals and convert them into digital form (using analog-to-digital conversion circuitry, ADC). To make this collection and conversion process more efficient, one or more columns of photo sensors are wired to share the same analog signal conditioning and multiplexing circuitry, and also the same ADC (generally referred to as column circuitry). In such a column-based approach, manufacturing variations from one column circuit to another will cause the resulting digitized image to exhibit vertical streaks (either bright or dark streaks), commonly referred to as vertical fixed pattern noise, VFPN, or column noise, even though the photo sensors may all be exposed to the same, uniform incident light. This, of course, degrades the quality of the digital image and therefore has received the attention of imaging system developers.
The noise or offset exhibited by a given column may be detected by computing the difference between an average photo sensor signal from one column and an ideal or expected value. This measure of the offset is then stored in non-volatile memory as an offset value, and may then be used to calibrate the imaging sensor by subtracting the offset value from the pixel column accordingly (each time the imaging sensor is used to take a picture).